1. Field of the Invention
This invention relates to a laser beam recording apparatus for recording a continuous tone image on a photosensitive material by scanning the photosensitive material with a laser beam modulated in accordance with an image signal. This invention particularly relates to a laser beam recording apparatus for recording an image of high gradation by analog modulation of the optical intensity of the laser beam.
2. Description of the Prior Art
Light beam scanning recording apparatuses wherein a light beam is deflected by a light deflector and scanned on a photosensitive material for recording an image on the photosensitive material have heretofore been used widely. A semiconductor laser is one of the means used for generating a light beam in the light beam scanning recording apparatuses. The semiconductor laser has various advantages over a gas laser or the like in that the semiconductor laser is small, cheap and consumes little power, and that the laser beam can be modulated directly by changing the drive current.
FIG. 2 is a graph showing the optical output characteristics of the semiconductor laser with respect to the drive current. With reference to FIG. 2, the optical output characteristics of the semiconductor laser with respec to the drive current change sharply between a LED region (natural light emission region) and a laser oscillation region. Therefore, it is not always possible to apply the semiconductor laser to recording of a continuous tone image. Specifically, in the case where intensity modulation is carried out by utilizing only the laser oscillation region in which the optical output characteristics of the semiconductor laser with respect to the drive current are linear, it is possible to obtain a dynamic range of the optical output of only approximately 2 orders of magnitude at the most. As is well known, with a dynamic range of this order, it is impossible to obtain a continuous tone image having a high quality.
Accordingly, as disclosed in, for example, Japanese Unexamined Patent Publication Nos. 56(1981)-115077 and 56(1981)-152372, an attempt has been made to obtain a continuous tone image by maintaining the optical output of the semiconductor laser constant, continuously turning on and off the semiconductor laser to form a pulsed scanning beam, and controlling the number or the width of pulses for each picture element to change the scanning light amount.
However, in the case where the pulse number modulation or the pulse width modulation as mentioned above is carried out, in order to obtain a density scale, i.e. a resolution of the scanning light amount, of 10 bits (approximately 3 orders of magnitude) when the picture element clock frequency is 1 MHz for example, the pulse frequency must be adjusted to be very high (at least 1 GHz). Though the semiconductor laser itself can be turned on and off at such a high frequency, a pulse counting circuit or the like for control of the pulse number or the pulse width cannot generally be operated at such a high frequency. As a result, it becomes necessary to decrease the picture element clock frequency to a value markedly lower than the aforesaid value. Therefore, the recording speed of the apparatus must be decreased markedly.
Also, with the aforesaid method, the heat value of the semiconductor laser chip varies depending on the number or the widths of the pulses which are output during the recording period of each picture element, so that the optical output characteristics of the semiconductor laser with respect to the drive current change, and the exposure amount per pulse fluctuates. As a result, the gradation of the recorded image deviates from the correct gradation, and a continuous tone image of a high quality cannot be obtained.
On the other hand, as disclosed in Japanese Unexamined Patent Publication No. 56(1981)-71374 for example, it has been proposed to record a high-gradation image by combining pulse number modulation or pulse width modulation with optical intensity modulation. However, also with the proposed method, the heat value of the semiconductor laser chip varies depending on the number or the widths of the pulses which are output during the recording period of each picture element, so that the exposure amount per pulse fluctuates.
In view of the above, in order to record a high-gradation image of a density scale of approximately 10 bits, i.e. approximately 1024 levels of gradation, it is desired that a dynamic range of the optical output be adjusted to approximately 3 orders of magnitude by carrying out optical intensity modulation over the LED region and the laser oscillation region as shown in FIG. 2. However, the optical output characteristics of the semiconductor laser with respect to the drive current are not linear over the two regions. Therefore, in order to control the image density at an equal density interval for a predetermined difference among the image signals so that a high-gradation image can be recorded easily and accurately, it is necsssary to make linear the relationship between the light emission level instructing signal and the optical output of the semiconductor laser by compensation of the optical output characteristics of the semiconductor laser with respect to the drive current.
As a circuit for making linear the relationship between the light emission level instructing signal and the optical output of the semiconductor laser, it has heretofore been known to use an optical output stabilizing circuit (an automatic power control circuit, hereinafter abbreviated to the APC circuit) for detecting the optical intensity of a laser beam and feeding back a feedback signal, which corresponds to the detected optical intensity, to the light emission level instructing signal for the semiconductor laser. FIG. 3 is a block diagram showing an example of the APC circuit. The APC circuit will hereinbelow be described with reference to FIG. 3. A light emission level instructing signal Vref for instructing the optical intensity of a semiconductor laser 1 is fed to a voltage-to-current conversion amplifier 3 via an addition point 2. The amplifier 3 feeds a drive current proportional to the light emission level instructing signal Vref to the semiconductor laser 1. A laser beam 4 emitted forward by the semiconductor laser 1 is utilized for scanning a photosensitive material via a scanning optical system (not shown). On the other hand, the intensity of a laser beam 5 emitted rearward from the semiconductor laser 1 is detected by a pin photodiode 6 disposed for optical amount monitoring, for example, in a case housing the semiconductor laser 1. The intensity of the laser beam 5 thus detected is proportional to the intensity of the laser beam 4 actually utilized for image recording. The output current of the pin photodiode 6 which represents the intensity of the laser beam 5, i.e. the intensity of the laser beam 4, is converted into a feedback signal (voltage signal) Vpd by a current-to-voltage conversion amplifier 7, and the feedback signal Vpd is sent to the addition point 2. From the addition point 2, a deviation signal Ve representing a deviation between the light emission level instructing signal Vref and the feedback signal Vpd is output. The deviation signal Ve is converted into a current signal by the voltage-to-current amplifier 3 and is utilized for operating the semiconductor laser 1.
In the case where the loop gain of the APC circuit as mentioned above is adjusted to be substantially high, the relationship between the light emission level instructing signal and the optical output of the semiconductor laser becomes linear.
The loop gain of the aforesaid APC circuit is determined by the gains of the amplifier included in the APC circuit, the photodetector and the semiconductor laser itself. As shown in FIG. 5, the gain, i.e. the differential quantum efficiency, of the semiconductor laser fluctuates in accordance with the optical output. Even though the differential quantum efficiency of the semiconductor laser fluctuates in this manner, the relationship between the light emission level instructing signal and the optical output of the semiconductor laser can be maintained linear in the case where the loop gain of the APC circuit is adjusted to be substantially high. However, on the other hand, the problems as described below arise.
Specifically, in the case where the differential quantum efficiency of the semiconductor laser is high, the gain of the APC circuit becomes high, the light emission response characteristics of the semiconductor laser become improved, and therefore the sharpness of the image is increased. However, in the case where the differential quantum efficiency of the semiconductor laser is low, the light emission response characteristics of the semiconductor laser are deteriorated, and therefore the sharpness of the image becomes low. Also, in this case, the relationship between the light emission level instructing signal and the optical output of the semiconductor laser cannot be maintained linear. In the case where a continuous tone image is recorded by modulating the light emission intensity of the semiconductor laser as mentioned above, a part exposed to the laser beam of a high intensity (a high density part in the case of a positive type photosensitive material) and a part exposed to the laser beam of a low intensity (a low density part in the case of the positive type photosensitive material) are naturally present in a single image. Therefore, when the differential quantum efficiency of the semiconductor laser fluctuates in accordance with the optical output as mentioned above, the sharpness changes between the high density part and the low density part in the image, and therefore the image quality is deteriorated.